1. Field of the Invention
The present invention is directed generally to wireless communication systems and, more specifically, to the transmission of control information in the uplink of a communication system.
2. Description of the Art
A conventional communication system includes a DownLink (DL), for supporting transmissions of signals from a base station (Node B) to User Equipments (UEs), and an UpLink (UL), for supporting transmissions of signals from UEs to the Node B. UEs, also commonly referred to as terminals or mobile stations, may be fixed or mobile and include devices such as wireless devices, cellular phones, personal computer devices, etc. A Node B is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or other similar terminology.
A UE typically transmits Uplink Control Information (UCI) to provide, to the Node B, information facilitating the communication process. The UCI may include ACKnowledgement (ACK) information associated with a Hybrid Automatic Repeat reQuest (HARQ), HARQ-ACK, Channel State Information (CSI), etc. A HARQ-ACK informs the Node B whether information was correctly or incorrectly received by a UE through a Transport Block (TB). A CSI informs the Node B of any of a Channel Quality Indicator (CQI), a Precoding Matrix Indicator (PMI), a Rank Indicator (RI), etc. A UE can transmit UCI separately from data information through a Physical Uplink Control CHannel (PUCCH) or, together with data information, in a Physical Uplink Shared CHannel (PUSCH) over a Transmission Time Interval (TTI).
The CSI includes the CQI, PMI, and RI and provides to the Node B information about channel conditions the UE experiences in the DL so that the Node B can select appropriate parameters, such as the Modulation and Coding Scheme (MCS), for a signal transmission to the UE and ensure a desired BLock Error Rate (BLER) for the respective information packet. The CQI provides, to the Node B, a measurement of the Signal to Interference and Noise Ratio (SINR) over sub-bands or over a whole operating BandWidth (BW), typically in the form of a highest MCS, for which a predetermined BLER for a signal transmission in the respective BW can be achieved. The PMI/RI includes information indicating, to the Node B, how to combine the signal transmission to the UE from multiple Node B antennas in accordance with the Multiple-Input Multiple-Output (MIMO) principle.
An example of a conventional PUCCH structure for the CSI transmission during an UL TTI, which for simplicity is assumed to consist of one sub-frame, is illustrated in FIG. 1. The sub-frame 110 includes two slots. Each of slots 120 and 125 includes NULsymb symbols for the transmission of CSI signals 130 or Reference Signals (RS) 140. Each symbol further includes a Cyclic Prefix (CP) to mitigate interference due to channel propagation effects. The location of the CSI transmission in the first slot 120 is at a different part of the operating BW than the CSI transmission in the second slot 125 in order to obtain frequency diversity.
Some symbols in each slot can be used for RS transmission to provide channel estimates and enable coherent demodulation of the received CSI signal. In the present example according to FIG. 1, the operating BW consists of frequency resource units referred to as Resource Blocks (RBs). Each RB consists of NscRB sub-carriers, or Resource Elements (REs), and a UE transmits CSI signals and RS over one RB 150 per sub-frame symbol.
An example of a conventional PUCCH structure for the HARQ-ACK transmission over a single sub-frame is illustrated in FIG. 2. Regarding the CSI PUCCH structure, the sub-frame 210 includes two slots and each of slots 220 and 225 includes NULsymb symbols for the transmission of HARQ-ACK signals 230 or RS 240. A UE transmits a HARQ-ACK signal and RS over one RB 250 and the transmission in the first slot 220 is at a different RB than the transmission in the second slot 225, in order to obtain frequency diversity. The HARQ-ACK PUCCH structure may have a different number of symbols in each slot for RS transmission and for HARQ-ACK signal transmission than the CSI PUCCH structure.
An example of a conventional structure for the CSI transmission in the PUCCH slot 120 of FIG. 1 is illustrated in FIG. 3. The transmission in the second slot 125 is at a different RB and has effectively the same structure. The CSI symbols d0,d1,d2,d3,d4 310 modulate 320 a “Constant Amplitude Zero Auto-Correlation (CAZAC)” sequence 330, for example, using Quaternary Phase Shift Keying (QPSK), which is then transmitted after performing an Inverse Fast Frequency Transform (IFFT), which is further described hereinbelow. Each RS 340 is transmitted through the unmodulated CAZAC sequence.
An example of a conventional structure for the HARQ-ACK transmission in PUCCH slot 220 of FIG. 2 is illustrated in FIG. 4. The transmission in the other slot 225 is at a different RB and has effectively the same structure. The HARQ-ACK bits b 410 modulate 420 a CAZAC sequence 430, for example with Binary Phase Shift Keying (BPSK) or QPSK, which is then transmitted after performing the IFFT. Each RS 440 is transmitted through the unmodulated CAZAC sequence.
An example of CAZAC sequences is given by Equation 1:
                                          c            k                    ⁡                      (            n            )                          =                  exp          ⁡                      [                                                            j                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                  k                                L                            ⁢                              (                                  n                  +                                      n                    ⁢                                                                  n                        +                        1                                            2                                                                      )                                      ]                                              (        1        )            where L is the length of the CAZAC sequence, n is the index of an element of the sequence n={0,1, . . . , L−1}, and k is the index of the sequence. If L is a prime integer, then L−1 distinct sequences are defined as k ranges in {0,1, . . . , L−1}. If an RB includes an even number of REs, such as for example NscRB=12, CAZAC sequences with even length can be directly generated through a computer search for sequences satisfying the CAZAC properties.
FIG. 5 illustrates an example of a conventional transmitter structure for a CSI or a HARQ-ACK in a PUCCH. The example according to FIG. 5 refers to a frequency-domain version of a computer generated CAZAC sequence, at block 510. A first RB and a second RB are selected, at block 520, and sub-carrier mapping is performed, at block 530, for transmission of the CAZAC sequence in the first slot and in the second slot, respectively. An IFFT is performed, at block 540, and a Cyclic Shift (CS), as it is subsequently described, is applied to the output, at block 550. Finally, the CP insertion, at block 560, and filtering, at block 570, are applied to the signal, which is transmitted at 580. In the example according to FIG. 5, a UE applies zero padding in REs used for signal transmission by other UEs and in guard REs (not shown). Moreover, for clarity and conciseness, additional transmitter circuitry such as digital-to-analog converter, analog filters, amplifiers, and transmitter antennas as they are known in the art, are not shown.
Reverse (complementary) transmitter functions are performed for reception of CSI or HARQ-ACK in the PUCCH. The reverse functions are conceptually illustrated in FIG. 6, where the operations are the reverse of the operations illustrated in FIG. 5. An antenna 610 receives the RF analog signal and after further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters) the digital received signal is filtered, at block 620, and the CP is removed at block 630. Subsequently, the CS is restored at block 640, and a Fast Fourier Transform (FFT) is applied at block 650. After sub-carrier de-mapping at block 660, the first RB and the second RB of the signal transmission in the first slot and in the second slot, respectively, are selected at block 665, and the signal is correlated at block 670 with a replica of the CAZAC sequence output from block 680. The output 690 can then be passed to a channel estimation unit, such as a time-frequency interpolator, in case of RS, or to a detection unit in case of CSI or HARQ-ACK.
Different CSs of the same CAZAC sequence provide orthogonal CAZAC sequences and can therefore be allocated to different UEs for PUCCH signal transmission in the same RB and achieve orthogonal UE multiplexing. This allocation principle is illustrated in FIG. 7. In order for the multiple CAZAC sequences 710, 730, 750, 770 generated correspondingly from the multiple CSs 720, 740, 760, 780 of the same root CAZAC sequence to be orthogonal, the CS value Δ 790 should exceed the channel propagation delay spread D (including a time uncertainty error and filter spillover effects). If TS is the symbol duration, the number of such CSs is equal to the mathematical floor of the ratio TS/D.
In addition to orthogonal multiplexing of PUCCH transmissions in the same RB from different UEs, as in FIG. 3 for a CSI or as in FIG. 4 for a HARQ-ACK, using a different CS of a CAZAC sequence, orthogonal multiplexing can also be achieved in case of HARQ-ACK in the time domain through the use of orthogonal covering codes. For example, in FIG. 4, the transmission of the HARQ-ACK information symbols may be modulated by a length-4 Orthogonal Covering Code (OCC), such as a Walsh-Hadamard code, while the transmission of the RS may be modulated by a length-3 OCC, such as a Discrete Fourier Transform (DFT) code (not shown for clarity and conciseness). In this manner, the multiplexing capacity of PUCCH transmissions in one RB in case of HARQ-ACK is increased by a factor of 3 (the factor being determined by the smallest of the OCC lengths).
For a UE transmitter equipped with more than one antenna, Transmitter Diversity (TxD) can provide spatial diversity and enhance the reliability of the received signal. Two such TxD methods for PUCCH transmission are the Space Time Block Coding (STBC) method and the Orthogonal Resource Transmission (ORT) method.
With STBC, considering the CSI, if the first antenna transmits the coded CSI symbols d0,d1,d2,d3,d4 in one slot, as shown in FIG. 3, the second antenna transmits the coded CSI symbols d1*,−d0*,d3*,−d2*,d4 where d* is the complex conjugate of d. Since the number of coded CSI symbols per slot is odd and the transmission in the first slot is at a different RB than the transmission in the second slot, one coded CSI symbol in each slot cannot be paired for STBC. In the above example, that coded CSI symbol was assumed to be d4 but any other symbol could have been selected. STBC maintains the use of only one CS for the CSI PUCCH transmission if the RS from the UE antennas in each slot are multiplexed using OCC where the OCC {1, −1} applies to the RS transmission from the second UE antenna. While FIG. 3 illustrates a CSI PUCCH transmission from the first UE antenna (in one slot), FIG. 8 is a direct equivalent for a CSI PUCCH transmission from the second antenna using STBC.
Denoted by hi,j the channel estimate for the signal received from the ith Node B antenna and transmitted by the jth UE antenna, where i=1,2 and j=1,2, and by yi,k the signal received by the ith Node B antenna in the kth symbol k=1,2, the decision for a pair of STBC symbols [{circumflex over (d)}k, {circumflex over (d)}k+1] is performed according to [{circumflex over (d)}k, {circumflex over (d)}k+1]T=HH [y1,k,y2,k,y1,k+1*,y2,k+1*]T where [ ]T denotes the transpose of a vector and
      H    H    =            [                                                                                    h                                      1                    ,                    1                                    *                                ⁢                                  h                                      2                    ,                    1                                    *                                            -                              h                                  1                  ,                  2                                            -                              h                                  2                  ,                  2                                                                                                                        h                                  1                  ,                  2                                *                            ⁢                              h                                  2                  ,                  2                                *                            ⁢                              h                                  1                  ,                  1                                            ⁢                              h                                  2                  ,                  1                                                                        ]                      (                                                                          h                                  1                  ,                  1                                                                    2                    +                                                                  h                                  2                  ,                  1                                                                    2                    +                                                                  h                                  1                  ,                  2                                                                    2                    +                                                                  h                22                                                    2                          )            .      
With ORT, the second UE transmitter antenna uses a second resource for the CSI transmission in the PUCCH. For example, for the CSI PUCCH transmission structure illustrated in FIG. 3, the first UE transmitter antenna uses a first CS and the second UE transmitter antenna uses a second CS. Therefore, the unpaired information symbol problem of STBC is avoided at the expense of doubling the number of required CSs, thereby doubling the respective PUCCH overhead. However, for HARQ-ACK signal transmission in the PUCCH, a UE may often have more than one orthogonal resource available (as determined by the pair of CS and OCC). For example, according to the 3rd Generation Partnership Project (3GPP) Re1.8 Long Term Evolution (LTE), a UE may have more than one resource available for HARQ-ACK signal transmission in response to reception of a dynamically scheduled TB. Then, ORT can be applied according to the structure illustrated in FIG. 4 without requiring additional resources beyond the resources a UE has already assigned.
A problem with the application of STBC for a CSI transmission such as that illustrated in FIG. 3 is the existence of an unpaired information symbol in each PUCCH slot. This unpaired information signal necessitates a different treatment of such information symbols, thereby increasing the transmitter and receiver complexity and diminishing performance, as the unpaired symbol cannot benefit from spatial diversity. Otherwise, STBC achieves full diversity gain without reducing the data rate and allows for a simple linear receiver. On the other hand, ORT requires twice the overhead, and other TxD methods, such as Cyclic Delay Diversity (CDD) or Precoding Vector Switching (PVS) provide worse performance relative to STBC or ORT.
Therefore, there is a need to enable the application of STBC for PUCCH transmissions while avoiding the problem of an unpaired information symbol. There is also a need to enable multiplexing of PUCCH transmissions with STBC and without STBC. There is further another need to optimize PUCCH reception performance with STBC or ORT.